After losing his father in Afghanistan, 18-year-old Seamus Donahue will run the Marine Corps Marathon in his honor

Seamus and Michael Donahue had a shared passion for running. They ran together whenever they got the chance. But Maj. Michael Donahue of the 82nd Airborne Division was killed in Afghanistan in 2014, and Seamus lost the man he calls his best friend. This Sunday, he'll run the Marine Corps Marathon to honor his father -- and so many others. My story for ESPN.com's Endurance Sports section: http://espn.go.com/sports/endurance/story/_/id/13947854/endurance-sports-seamus-donahue-honoring-fallen-father-marine-corps-marathon